A bug called "Update the default wallpaper in 13.04" has been reported on Launchpad today, revealing the new Ubuntu 13.04 wallpaper which, like we're used to, is just a slightly modified version of the previous default wallpaper:

But the bug report I was talking about revealed something a lot more interesting than this: it links to bugreports about some new icons which will be used for Ubuntu Software Center, Nautilus, Software Updater and some launcher icons like the BFB icon (the Launcher icon that's used to open Dash), or the workspace switcher (the new icon should change depending on which workspace is currently being utilised, but this hasn't been implemented in Unity yet) which you can see below:

Since the icons don't exactly express what they do, let me explain: the icon in the middle of the ALT+TAB dialog in the screenshot above is the new Ubuntu Software Center icon, the one on the right of the USC icon is the Software Update icon (the green icon). And the icon on the left of USC, as you've probably figured out, is the Nautilus icon.

The new wallpaper / icons have not landed in Ubuntu 13.04 yet and are subject to change.